1. Field of the Invention
This invention generally relates to processing signals, and more particularly to a system and method for generating analog signals for transmission in a communications, a photonics-based system, or any other system which requires analog signal processing.
2. Background of the Related Art
In the broadest sense, a communications transmitter operates by modulating a carrier wave with an input (e.g,. voice) signal. The modulated signal is then transmitted at a predetermined frequency to a receiver, where it is detected and demodulated so that the original signal may be recovered. The recent demand for integrated voice and data services has given birth to new modulation schemes designed to make the most efficient use of bandwidth, while simultaneously achieving fast transmission rates. Today, these modulation schemes are implemented in both digital and analog form.
Analog transmissions systems have been performed using a combination of fiber-optic and wireless technologies. Conventional systems of this type convert information-bearing RF signals into amplitude-modulated optical signals. These so-called analog fiber-optic links have been shown to have great advantages over microwave links in terms of their bandwidth and immunity to electromagnetic interference. These links also require smaller and lighter their hardware than their microwave-counterparts.
For all their advantages, analog fiber-optic links have at least one drawback which significantly degrades their performance. This drawback comes in the form of a large insertion loss, which results from an inefficient conversion of RF signals to amplitude-modulated optical signals.
Several approaches have been taken in an attempt to overcome the drawbacks of analog fiber-optic links. One approach involves the use of high-saturation power detectors and low V-π modulators. This approach has proven undesirable because a balancing scheme must be used which increases the complexity of the system. This approach is also deficient because an improved weakness of loss is required without introducing degradations to other figures of merit such as noise figures and spurious-free dynamic range (Drsp).
Another approach, disclosed in U.S. Pat. No. 6,331,991, converts a frequency-modulated input signal into an amplitude-modulated signal prior to transmission. This conversion is performed using a transfer function which assigns frequency variations in the input signal to one of two discrete amplitude values. It is clear that the approach taken in this patent therefore uses a non-linear transfer function which performs a digital conversion. A transfer function of this type has proven undesirable for many communications applications involving analog links.
In view of the foregoing discussion, it is clear that there is a need for a system and method which generates analog transmission signals without realizing the drawbacks of conventional systems, and moreover which does so without realizing an insertion loss and without using a non-linear transfer function which has been shown to be unsuitable for use in many analog transmission applications.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.